Conventional energy is being replaced by renewable energy because of the energy crisis and the environment pressure. The renewable energy such as wind energy and solar energy and the like has been developed in large scale. However, the impacts to the electricity power grid due to the instability of such kind of energy are getting worse and worse. Therefore, it is necessary to research and develop a high capacity energy storage system, which is low-cost and has high-efficiency, for load-shifting to obtain a stable renewable energy. Among a number of energy storage systems, redox flow battery has been developed intensively because of its advantages of adjustable capacity, free of solid phase reaction, free of change of the electrode material microstructures, low cost, long life, high reliability, and low cost for operation and maintenance.
Vanadium redox flow battery (hereafter referred to as VRB) is a renewable battery energy storage system based on the redox reaction of metal element vanadium. In a vanadium battery, electricity energy is stored in sulfate electrolyte of vanadium ions of different valences in the form of chemical energy. The electrolyte is forced into the battery stack by an external pump and thus is circulated in a closed circuit comprised of different storage tanks and half cells. With a proton exchange membrane (PEM) which serves as a separator of the battery, electrolyte solutions flow in parallel across the surfaces of electrodes and an electro-chemical reaction occurs. Electricity current is gathered and conducted by bipolar plates. In this way, the chemical energy stored in the electrolyte solutions is converted into electricity energy. Such a reversible reaction enables the vanadium battery to charge, discharge, and recharge smoothly.
However, during charge/recharge cycles of the VRB, the migration of ions and water between a positive electrode and a negative electrode causes the electrolytes to be out of balance gradually, and thus the efficiency and the capacity of the battery is decreased, as occurred in other kind of redox flow batteries.
In order to solve the problem, a complex procedure is necessary to mix the positive and negative electrolytes to an initial state after a period of operation. Such a procedure is quite complex and needs additional electricity power to perform the mixing procedure.
With respect to the conventional process, U.S. Pat. No. 6,764,789 discloses two substitutive methods: the batchwise liquid adjusting method and the overflow method. The batchwise liquid adjusting method is performed by pumping the positive or negative electrolyte in storage tank whose liquid level has raised into the negative or positive electrolyte in storage tank whose liquid level has lowered after several (e.g., 30) charge/discharge cycles, and the overflow method is performed by setting an initial level difference between the positive electrolyte storage tank and the negative electrolyte storage tank and allowing the increased electrolyte in one of the positive electrolyte storage tank and the negative electrolyte storage tank whose liquid level has raised to flow into the other one whose liquid level has lowered through a pipe connecting both tanks of the positive electrolyte and the negative electrolyte with the aid of gravity.